The invention relates to a predistortion control device. The invention further relates to an assembly including a predistortion control device, and to a predistortion control method. The invention also relates to an electronic device including a predistortion control device.
Power amplifiers are generally known in the art. Power amplifiers are used, for example, in wireless transmission systems to amplify a signal such that the signal has sufficient energy to be transmitted via an antenna. However, often the performance is limited by the non-linear behaviour of the Power Amplifier (PA). In an ideal PA, the amplitude of the output signal is a linear multiplication of the amplitude of the input signal, and therefore equal to the input amplitude times a multiplication constant, i.e. described mathematically:G(x)=Const*x,
in which G(x) represents the output signal, x the input signal, and Const represents the multiplication constant. However, in reality every power amplifier has a certain deviation of the ideal PA. The deviation causes the presence of non-linear components in the output signal, i.e. described mathematically:G(x)=Const*x+f(x),
with f(x) representing a non-linear function of the input signal x.
The effect of this non-linear behaviour is spectral distortion. If the input signal before (power) amplifications is band-limited (concentrated within a specified frequency range), the signal at the output of the Power Amplifier will have a significant amount of energy outside the specified frequency range of the input signal. Telecommunication standards like GSM and UMTS specify the maximum amount of energy present outside the specified frequency range and thereby implicitly impose a maximum to the ‘non-linearity’ of Power Amplifiers.
In the art, several linearisation techniques are known to counteract the non-linearity of a PA, and in order to provide a linearisation of Power Amplifiers at real time. A first technique is feed forward. In this technique, an estimate of the distortion by the non-linearity of the PA is obtained. Based on the estimate, a correction signal is subtracted from the amplified signal presented at the output of the PA, in order to correct for the non-linear components present in the amplifier signal. Another technique is feedback. In this technique, the difference between the input signal of the power amplifier and the output signal thereof is minimised by means of a suitable control loop.
Furthermore, predistortion systems are known. Predistortion systems, include a predistortion device which adds, from a mathematical point of view, a predistortion signal to the input signal of the power amplifier. In practical systems, a look-up table can be used which maps an input sample onto an output sample. The combined input signal is presented to the power amplifier input. The predistortion signal is such that the output of the PA resulting from the combined predistortion signal and the original input signal approximates the output of an ideal power amplifier for the original input signal as good as possible.
The predistortion device is set by means of a predistortion control device, which determines from time to time an approximation of the non-linearity of the power amplifier. The predistortion control device provides to the predistortion device a predistortion control signal representing the approximation, to correct for example for changes in the behaviour of the power amplifier due to thermal changes or aging of the power amplifier.
However, a draw-back of these known linearisation techniques is that stringent requirements are imposed on the devices used in the linearisation, because the analogue components, such as analog-to-digital converters and frequency converters, need to be highly linear and stable over a relatively large frequency range. These requirements limit the performance of the linearisation techniques.
Predistortion systems have the additional draw-back that the processing involved with the approximation of the non-linearity of the power amplifier is complex, and has to be executed at the sample rate of the predistortion control device. Predistortion systems therefore have a relatively high power consumption and are costly to implement.